1. Field of the Invention
The present invention is directed generally to methods for detecting and quantifying leakage from storage tanks. The present invention is more specifically directed to improved techniques for continuous and automatic calibration of systems designed to detect and quantify leakage from liquid storage tanks and their lines to the meters, particularly those used with petroleum based products.
2. Description of the Prior Art
There are two related problems that occur in the management of large quantities of liquid product within above ground or underground storage tanks. The first of these problems involves simple inventory control and tracking of the quantity, level, and condition of the product within the tank. The second is a related problem and involves the detection of any unintentional releases of the liquid product from the tank most commonly through the existence of a leak. Both of these problems involve the accurate, real time, determination of the quantity of the product within the tank and the change in this quantity over periods of time. Because of cost controls and environmental concerns, it has become increasingly important in recent years, to very accurately determine and control industrial and other liquid products contained within storage tanks.
It is well known in the field to use both level monitors and flow meters to determine the quantity of a liquid product within a tank and/or to determine the amount of product that is dispensed from the tank by a user. The use of tank level meters, even accurate ones, typically involves a number of assumptions about the characteristics of the liquid product and the environmental characteristics such as temperature and pressure. The use of a level meter in conjunction with what is referred to as a tank "strap chart" is intended to provide the user with an indication of the quantity of the liquid remaining in the tank and, therefore, to some extent the quantity of liquid that might have been dispensed from the tank. A tank strap chart is a manufacturer's calculation or determination of the amount of liquid that would exist in the tank if a level meter in a particular position in the tank registered a specific level. Because the tanks are typically cylindrical in structure, a tank strap chart might either be determined based upon experimentation or based upon a mathematical model of the approximate dimensions of the tank. In any case, regardless of the actual shape of the tank, the combination of a level meter and a tank strap chart could, for example, allow the user to determine how much product was dispensed from the tank when the level dropped from 4.8 feet to 4.5 feet according to the level meter. Of course, that dispensed quantity would be different from the amount reflected by a drop from say 1.5 feet to 1.2 feet, even though the vertical level change is the same.
The problems associated with this relatively straight forward method of determining the quantity of liquid product in the tank and the amount that might have been dispensed, involve the impropriety of making assumptions about the effects of temperature and pressure and variations in the specific gravity of the product that might be placed within the tank. Temperature, pressure, and specific gravity all affect the volume per mass value for a particular product and, therefore, as these factors change the actual quantity of liquid product in the tank might be inaccurately measured.
As a rough example, a given quantity of petroleum product within a storage tank will register at one level when the temperature is at a cooler value and register at a slightly higher level when the temperature is warmer. In addition to temperature affecting the volume of the liquid product, the enclosed tank is actually an equilibrium environment in which the product exists in both a liquid state and a vapor state in the "ullage" or space above the liquid product. This equilibrium constantly transfers products from liquid state to vapor state and affects the actual quantity of the product measured by a level meter within the tank, especially when the product is being dispensed from or placed into the tank.
Finally, the method of simply using a tank strap chart and a level meter for determining the quantity of liquid product within a tank involves a number of inaccuracies associated not only with the level variations that are due to temperature, etc., but also with the assumption that the tank strap chart continues to be accurate over time. Even if a tank strap chart is accurate at the time it was created, most likely by the manufacturer immediately after sale, chances are that the tank itself has become deformed or dented or misconfigured in some way. Any alteration of the three dimensional configuration of the tank can significantly affect the accuracy of the tank strap chart. Failure to keep these inaccuracies in mind could reduce the final accuracy of any measurements associated with inventory control of the tank product. Even more importantly when the user becomes concerned with more difficult to measure changes in the product level such as those caused by leakage, these inaccuracies could make the possibility of detecting a leak altogether impossible.
Efforts to supplement the above methods of inventory and quantity control for a liquid product within a tank by more accurately measuring the flow of the product in and out of the product of the tank have helped, but have by no means solved the problems. In the typical application of a petroleum product liquid storage tank, namely the neighborhood gas station, gasoline products may be continuously being dispensed from a tank during 24 hour day. Intermittently, the tank is refilled by the placement of a large quantity of fuel from a delivery tank. Theoretically, accurate measurements of flow into and out of a tank could allow the station manager or owner to determine whether any fuel was leaking from the tank in an uncontrolled manner. Obviously if it took more to refill the tank from a delivery tank than was dispensed from the tank and measured by way of flow meters in the station's gasoline pumps, then there would be some indication that leakage from the tank was occurring. Even here, however, there is some need to accurately determine a specific level within the tank so as to define when the tank is full upon delivery and then filled to that level again upon delivery.
Temperature variations and differences in the characteristics of liquids dispensed also affect this method of measuring and inventory control. Even more importantly, however, are inaccuracies that exist in the various flow meters used for both dispensing liquid product from the tank and placing the liquid product within the tank upon delivery. Although these meters are calibrated, in many cases as required by law, these calibrations do change not only with respect to individual meters and over time, but with respect to temperature, pressure, and the characteristics of the liquids being pumped.
Even the combination, therefore, of a tank level measuring means and a flow meter arrangement has a number of inherent inaccuracies that result from factors that are not typically addressed in an analysis and determination of inventory quantity. Such systems are capable of measuring rather significant anomalies in inventory that might result from large leaks, but are not capable of accurately determining inventory so that small leaks can be detected and more accurate control of the product can be achieved.
There is a significant need, therefore, especially in the area of petroleum product storage, for a method and system for accurately detecting and quantifying leakage from underground and above ground storage tanks. It is important to take into consideration all factors that are known to affect the ability of existing technology to make accurate level and flow measurements and to collect level and flow data and analyze it in conjunction with known liquid characteristics. This would accurately characterize the controlled inflow and outflow of product from the tank not only for the purpose of controlling costs and monitoring inventory, but perhaps more importantly for the purpose of detecting and immediately quantifying leakage of sometimes toxic liquids from storage tanks.